The use of actinic radiation, such as ultraviolet radiation, to cure ink compositions is generally known in the art. Ultraviolet radiation can be used to cure various types of inks, such as thiolene inks, inks made up of aryl diazonium salts and epoxy resins, and inks containing acrylates including acrylated epoxies and urethanes. Of these, acrylate containing inks are often preferred because they are available at a reasonable cost and have good storage stability in addition to their useful properties as inks.
Acrylate type UV curable inks are typically made up of a pigment dispersed in a reactive base that may contain photoinitiators, reactive monomers or oligomers, preservatives, flow agents, and so on. The properties of the ink such as viscosity, gloss, and crosslink density can be controlled by varying the types and/or proportions of reactive diluents used in the formulation.
Radlove et al., in U.S. Pat. No. 3,856,744 describe photopolymerizable ink compositions that comprise an ethylenically unsaturated beta-hydroxy ester, a polyitaconate, and optionally a polyacrylate which functions as a reactive diluent. The Radlove compositions must contain 2 to 10 wt-% of the polyitaconate.
In U.S. Pat. No. 4,826,890 Gupta teaches radiation curable coating compositions that comprise a copolymer of dialkyl muconate and p-acetoxystyrene, a polyethylenically unsaturated radiation polymerizable compound, and optionally a monoethylenically unsaturated monomer such as an acrylate or methacrylate. Additional examples of radiation curable coatings are found in Wojnarowicz, EP 457 499; Kistner, U.S. Pat. No. 4,668,601; Liu et al., U.S. Pat. No. 5,106,885; and Hesse et al., U.S. Pat. No. 4,265,723.
Macromonomers, also known as high molecular weight monomers, are large molecules made up of repeating monomeric units and terminated by a polymerizable end group. These large monomers have long been used in the preparation of graft copolymers to provide uniform side chains or branches. Milkovich, U.S. Pat. Nos. 3,786,116 and 3,842,059 are examples of the use of macromonomers in the synthesis of graft copolymers. Acrylate containing macromonomers are known, as seen for example in Mori, U.S. Pat. No. 5,147,952.
Attempts have been made to incorporate macromonomers into UV curable inks. In EP 187,045, Kerr teaches the use of macromonomers in the preparation of radiation curable printing inks. The macromonomers used by Kerr have a repeating unit that is a vinyl aromatic monomer. Properties such as storage stability, adhesiveness, durability and so on are attributed by Kerr to the large mass of vinyl aromatic in the macromonomer. However, compositions containing high proportions of an aromatic monomer can have certain undesirable properties. For example, these compositions may be vulnerable to degradation by ultraviolet light, resulting in poor durability and short product life for applications where significant outdoor exposure is required. Aromatic macromonomer containing ink compositions may not adhere well to non-aromatic polymer substrates such as polymethylmethacrylate, making them less well suited to use in constructions containing such materials.
A need exists for radiation curable ink compositions that are durable, resistant to ultraviolet light once cured, and that adhere well to acrylate substrates.